1.0 Field of the Invention
The invention relates to power supplies and, more particularly, to a booster circuit for foldback current limited power supply that prevents the foldback current limited power supply from operating during its turn-on phase.
2.0 Description of the Prior Art
Modern power supply engineering design practice requires that provision for short circuit protection be present. Foldback current limiting is a form of control that has the advantage of reducing a short circuit current to considerably less than a predetermined maximum current that can be supplied by the power supply. Power supplies having foldback current limiting and/or current limiting capabilities are well known and some of which are described in the following nine (9) U.S. Patents, all of which are herein incorporated by reference: U.S. Pat. Nos. 3,345,554; 3,723,774; 3,753,079; 4,587,476; 4,658,201; 4,660,122; 4,791,544; 5,578,916 and 5,666,044.
The result of foldback current limiting is that the maximum power dissipation of the power supply components, which occurs during short circuit conditions, is reduced. The reduced maximum power dissipation allows for smaller power transistors and heat sinks which translates into reduced power supply size and cost. However, the foldback current limiting technique has its disadvantages.
Power supplies incorporating foldback current limiting cannot attain their rated output voltage when presented with a predominantly constant current load and when this load is any reasonable fraction of the full rated output current of the power supply. Constant current loading is prevalent in the design practice as many analog circuits of the operational amplifier type present this load. Therefore, any design that incorporates a large array of operational amplifiers of this type, and where this array is a predominant portion of the total load of the power supply, and where this load makes efficient use of the power supply by being a reasonable fraction of the maximum power rating of the power supply, needs to incorporate special provisions for dealing with a foldback current limiting. These special provisions may be further described with reference to FIG. 1.
FIG. 1 depicts the model operating characteristic of a foldback current limited power supply. FIG. 1 represents a graph of output voltage, V.sub.O, versus output current I.sub.O ; time is not a factor in FIG. 1; it is merely a depiction of the various steady state operating conditions of the foldback current limited power supply. During normal operation, the power supply operates along the portion of the characteristic between points A and B. This is to say that the foldback current limited power supply will regulate the output voltage at a constant predetermined value of V.sub.O for any load current ranging from zero to a maximum rated value denoted as I.sub.MAX in FIG. 1.
If the load develops a fault that tends to draw current in excess of I.sub.MAX, the foldback current limited power supply will sense that its output is at point B and will reduce the output current in a manner illustrated by segment B-C of the characteristic of FIG. 1. The foldback current limited power supply will attain a steady state on characteristic B-C at a point determined by the V versus I characteristic of the fault load. A pure short circuit, i.e., a fault which develops zero voltage drop at any load current will cause the supply to reach and maintain operation at point C also indicated by short-circuit current I.sub.SC. In addition to characterizing the foldback current limited power supply operation during fault conditions, the model curve of FIG. 1 is used to predict the foldback current limited power supply operation during its turn-on phase. The load conditions may be further described with reference to FIG. 2.
FIG. 2 illustrates the characteristics of FIG. 1, but in addition thereto, shows the model operating characteristic with two types of load lines drawn in dashed lines and labeled I and II. In FIG. 2, unlike FIG. 1, time is a factor for the dashed trajectories, that is Type I and II loads, in that it is understood that the load will change in the direction of the arrowheads of the X and Y axes as time proceeds and the voltage across the load increases.
The type I load is characteristics of a predominantly resistive load and lies within the area defined by the operating characteristic O-A-B-C. Type I is the type of load for which the foldback current limited power supply is designed and will not cause turn-on problems as it traverses segment O-F which, as seen in FIG. 2, does not cross the current limit segment C-B. The Type II load is typical of a load which is predominantly constant current. During turn-on, the type II load will attempt to traverse the segment from O to D to E. The power supply foldback limiting will catch the load at point D and maintain the output at the output voltage denoted V.sub.F in FIG. 2. The foldback current limited power supply is now said to be hung at V.sub.F and will not come up to full output voltage V.sub.O at point E which is well within the operating characteristics of the power supply. This hang-up at V.sub.F is then the crux of the problem. It is desired to create corrective means by which the foldback limit operation of the foldback current limited power supply is temporarily suspended during turn-on so that nonlinear loads, that is, those considered to be Type II loads, that are well within the rated operating characteristic of the supply can be powered to the full output voltage, V.sub.O ; and at the same time retain the foldback limiting during normal operation so that the benefits of short circuit protection are retained.